Following the increase in demand for data communications, high-speed large-capacity downlink packet systems have been actively studied. For example, in the next generation mobile communication system (IMT-2000), the high speed downlink packet transmission (HSDPA: High Speed Downlink Packet Access) using the W-CDMA (Wideband-Code Division Multiple Access) has been discussed in the 3GPP (3 rd Generation Partnership Project).
In the HSDPA, a high speed downlink shared channel (HS-PDSCH: High Speed-Physical Downlink Shared Channel) is used for downlink transmission from a base station to a mobile station. The HS-PDSCH is for transmitting packet data, and a plurality of mobile stations can use one HS-PDSCH in a shared manner by temporally sharing (time-sharing) it.
In the HSDPA system, uplink control channels (HS-DPCCH: High Speed-Dedicated Physical Control Channel) are set between a base station and a plurality of mobile stations for controlling data transmission from the base station to the mobile stations. The HS-DPCCH is used by the mobile station for transmitting ACK/NACK information about an HARQ (Hybrid Automatic Repeat reQuest) and communication path quality information to the base station.
The communication path quality represents a signal power to interference power ratio (SIR: Signal to Interference Ratio) of a common pilot signal (CPICH: Common Pilot Channel). Here, since all channels are temporally multiplexed and transmitted, the mobile stations each can use the common pilot channel transmitting known data symbols, so as to measure the reception quality.
The HSDPA system uses the AMCS (Adaptive Modulation and Coding Scheme) that adaptively changes modulation and coding rate according to a notified communication path quality. When the AMCS is applied, it is possible to carry out transmission that depends on the communication path quality. Specifically, when the communication path quality is excellent, the throughput can be improved by applying a modulation system with a large multilevel value and an error correcting code with a large coding rate while, when it is poor, the packet error rate can be suppressed by reducing both the multilevel value and the coding rate, so that it is possible to increase the system capacity.
In the packet transmission like the HSDPA, after receiving data transmission requests from a plurality of mobile stations, a base station determines transmission order among the mobile stations (scheduling) and transmits data. In this scheduling, use is made of communication path qualities notified from the mobile stations. The scheduling that performs packet transmission preferentially to such a mobile station with a high communication path quality is called a Maximum C/I scheduler.
When the Maximum C/I scheduler is used, transmission is performed in an instant when the communication path quality is high. Therefore, when the AMCS is applied, the probability of selecting a higher MCS level increases to thereby raise the average value of transmission rates, so that it is possible to increase the system throughput.
Further, apart from the communication system, an adaptive antenna enables separation of signals by the use of its directivity and, when applied to downlink communication, enables reduction in interference. Therefore, by applying the adaptive antenna to a downlink common channel that transmits packet data, the power can be transmitted convergently only in the direction of a mobile station so that it is possible to reduce interference to other users.
Conventionally, in case where the adaptive antenna is not applied to downlink transmission, the communication path quality observed by a mobile station does not rely on the state of communication. However, when the adaptive antenna is applied thereto, since a data channel is transmitted only in the direction controlled by the directivity, although the data channel is subjected to multipath interference while packet transmission is carried out, the data channel is not subjected to multipath interference while packet transmission is not carried out. This is because data channels of other mobile stations are separated by the directivity so that interference power is reduced. That is, the communication path quality observed by the mobile station relies on the state of communication.
In the foregoing conventional mobile communication system, the common pilot channel is used as a channel for estimating the communication path quality that is notified to the base station from the mobile station. However, there arises a problem that since the common pilot channel is not transmitted to a particular mobile station with a directivity given thereto, the communication path differs from a channel that actually transmits packets so that there occurs a difference between the estimated communication path quality and a communication path quality upon reception.
Further, a dedicated control channel controlled in directivity and given to each of users that carry out packet communication may be used for communication path quality estimation. However, if the control channels are allocated to all users in communication and on standby to carry out the communication path quality measurement, signal power to be allocated to the users on standby, which is not primarily necessary, increases and this causes interference to the users receiving packets. Thus, there is no merit of applying the adaptive antenna. Alternatively, if, as conventional, dedicated control channels controlled in directivity and given to only those users receiving packets are used for the communication path quality estimation, there is a problem that the communication path quality estimation cannot be carried out while waiting for packets.
Further, in the foregoing conventional mobile communication system, although MCS selection is carried out based on the communication path quality, when the adaptive antenna is used, the data channels of other mobile stations are separated in a standby state by the directivity of the adaptive antenna, and therefore, it is not possible to know degradation of the communication path quality due to multipath interference to the data channel in a communication state.
Consequently, in the conventional mobile communication system, there arises a problem that the communication path quality measured in the standby state becomes higher than the communication path quality in the communication state, and therefore, if MCS selection is carried out based on the communication path quality measured in the standby state, the probability of occurrence of packet reception errors becomes high so that the transmission efficiency is reduced.
Further, in the conventional mobile communication system, it is important to use the communication path qualities measured in the same condition over all mobile stations when carrying out the scheduling, but, when the adaptive antenna is used, the communication path quality differs depending on the communication state of the mobile station.
Conventionally, the interference power used for estimation of the communication path quality is not measured instantaneously, but the average value is derived over a predetermined time. The communication path quality becomes poorer with respect to such a mobile station that performs packet transmission at a higher time rate in the predetermined time. That is, the communication path quality differs depending on the communication state (the rate of time in which packet transmission is carried out). Therefore, in the conventional method, there arises a problem that it is not possible to perform the comparison in the same condition.
Further, since interference to the data channel from other mobile stations is small in the standby state by means of the directivity, the communication path quality is measured better than in the communication state. When there exist a mobile station in the standby state and a mobile station in the communication state at the same time, the communication path quality notified by the mobile station in the standby state becomes high while the communication path quality notified by the mobile station in the communication state becomes low.
In this event, the Maximum C/I scheduler transmits data to the mobile station with the high communication path quality to bring it into the communication state and stops data transmission to the mobile station with the low communication path quality to bring it into the standby state. At the next time, the states are switched between these mobile stations. In the state where the states are further switched, the scheduler changes the states of the mobile stations by using notified communication path qualities, and therefore, there occurs a phenomenon that the standby state and the communication state are alternately switched between the mobile stations.
Here, since the MCS level of the mobile station brought into the communication state is determined on the basis of the communication path quality in the standby state, the probability of occurrence of packet errors increases, which further brings about an increase in the number of times of retransmission. Consequently, there arises a problem that the system throughput is reduced.
It is an object of the present invention to provide a mobile communication system, a mobile station, a base station, and a communication path quality estimation method for use therein that can improve the system throughput.
Further, it is another object of the present invention to provide a mobile communication system, a mobile station, a base station, and a communication path quality estimation method for use therein that can estimate a communication path quality in a constant condition that does not rely on a communication state immediately before.